1. Field of the Invention
The present invention relates to the biosynthesis of an antigen of human Hepatitis B virus (HBV) by yeast, brought about by an application of recombinant DNA techniques.
2. Description of the Related Art
Hepatitis B virus is recognized as a major, worldwide public health problem. In addition to the widespread incidence of viral hepatitis, and the persistence of asymptomatic carrier states, Hepatitis B virus has been implicated in the etiology of hepatocellular carcinoma. For a recent review of the molecular biology of Hepatitis B virus, see Tiollais, P., et al., Science 213, 406 (1981).
A major effort in current research is to produce a suitable vaccine to provide protective immunity against viral infection. One line of approach to preparing a suitable vaccine has involved attempts to purify the principal antigenic component of the virus, the surface antigen. Hereinafter, the symbol HBsAg is used to identify HBV surface antigen obtained from preparations of intact virus (Dane particles) or purified from the serum of hepatitis carriers. Skelly, J. et al., Nature 290, 51 (1981) have reported the purification of water-soluble protein micelles of purified HBsAg. A significant limitation of this approach is that the amount of material which can be prepared depends upon the availability of donors. No technique is known for growing the virus in culture; therefore, in addition to limitations in the amount of source material, there is a risk of contamination of the vaccine with active virus or other components of donor serum, and a possible heterogeneity in the products obtained form various donors.
A second approach has been the attempt to synthesize peptides eliciting antibodies against HBsAg based upon the amino acid sequence of the protein comprising the surface antigen (S-protein) and model studies predicting the most likely antigenic determinants. See, e.g. R. A. Lerner et al., Proc. Nat. Acad. Sci. USA 78, 3403 (1981). Such work is in a highly preliminary stage, and it may be difficult to assess whether the approach can produce antigens having a practical degree of immunogenicity in a cost-effective manner.
A third approach, employing recombinant DNA techniques, is the synthesis of S-protein, HBsAg or an immunologically reactive equivalent by a microorganism, by endowing a microorganism with genetic capability to produce S-protein, HBsAg or an immunologically reactive equivalent in large amounts, in the absence of other viral gene products. This approach eliminates the possibility of contamination by virus or other viral components and permits large-scale production with economies of scale. Furthermore, it is possible, through appropriate manpulations of the genetic material, to modify the sequence of the protein comprising the vaccine, in order to modify its side effects, or make the vaccine polyvalent. Toward this end, the entire genome of HBV has been cloned in E. coli and its entire nucleotide sequence determined (Charnay, P., et al., Nucl. Acid Res. 7, 335 (1979); Galibert, F., et al., Nature 281, 646 (1979); Valenzuela, P., et al., Animal Virus Genetics (B. Fields, R. Jaenisch and C. F. Fox, Eds.) Academic Press, New York, N.Y. (1980), page 57. A single region of the genome was found to code for the S-protein and also for a large pre-sequence of 163 amino acids. The structure of HBsAg is believed to consist of two S-protein chains joined by intermolecular disulfide bonds and held in a prescribed confirmation by additional intra-molecular disulfide bonds. One of the two chains appears to be glycosylated. In the serum of carriers, HBsAg frequently appears in the form of spherical particles with a mean diameter of 22 nm, which are thought to aggregates of the S-protein dimers just described, and possibly contain lipids. In the viral envelope, HBsAg is associated with the lipid-containing viral envelope, which is believed to be derived from membrane components of the host cell.
The antigenicity and immunogenicity of HBsAg depend upon several factors, not all of which are well understood. It has been observed that reduction of the disulfide bonds reduces antigenicity and immunogenicity markedly (Mishiro, S. et al., J. Immunol. 124, 1589 (1980)). Therefore, the tertiary configuration contributed by the intramolecular and intermolecular disulfide bonds is thought to contribute to antigenicity and immunogenicity. The contribution of other factors, such as the extent and nature of glycosylation and association with lipid is unclear, although all are thought to contribute to some degree. Aggregation into particles such as the above-mentioned 22 nm particles is thought to contribute significantly to enhancing immunogenicity.
The S-protein has been synthesized in E. coli in the form of a fusion protein (Edman, J. C. et al., Nature 291, 503 (1981)). The product included 183 amino acids of pre-beta lactamase, 5-10 glycine residues, and 204 amino acids of S-protein lacking 22 amino acids of the amino terminal end. The fusion protein was immunoprecipitable with anti-HBsAg IgG.
Since it is known that S-protein dimers Mishiro et al., supra) and 22 nm particles incorporations HBsAg (Cabrall, G. A. et al., J. Gen. Virol. 38, 339 (1978)) are more antigenic than the associated S-protein, it would be highly desirable to find a biological system capable of producing HBsAg or an immunologically reactive equivalent directly, in substantial quantities.
The steps in converting S-protein to HBsAg or to 22 nm particles are not fully understood, nor is it known to what extent they are host cell-specific. Furthermore, the S-protein gene appears to code for an unusually long pre-sequence of 163 amino acids, whose functional significance, if any, is unknown. In fact it is not known whether the pre-sequence is actually translated in the virus-infected cell. Yeast (Saccharomyces cerevisiae) was chosen as a host cell in which to attempt the expression of HBsAg for the following reasons: Yeast is readily grown in culture in large quantities. In fact, the technology of yeast culture on a large scale is well understood. Also, yeast is eucaryotic, so it was hoped that some of the post-translational processing steps which are carried out in a normal host cell might he carried out in yeast. Because of the complex post-translational events that convert S-protein to HBsAg, some of which may be host-cell specific, the nomenclature adopted herein is intended to distinguish different antigenic forms recognized from the work herein disclosed. The unprocessed translation product of the structural gene for surface antigen is termed S-protein. The antigen isolated from plasma of infected donors, from Dane particles or from human hepatoma cell cultures, is termed HBsAg. The expression product of the surface antigen gene in yeast is termed Y-HBsAg. The term, immunologically reactive equivalent of HBsAg, is a general term for any immunologically cross-reactive composition comprising S-protein or a portion thereof, of which Y-HBsAg is an example.
Yeast has never previously been used for expression of the genes of a virus which normally multiplies in a different organism. Prior art attempts to express heterologous proteins in yeast have yielded mixed results. An attempt to express rabbit globin, under control of its own promoter, appears to have been unsuccessful in translation of the protein (Beggs, J. D. et al., Nature 283, 835 (1980)). A gene coding for a Drosophila gene has been reported capable of complementing a yeast ade 8 mutant, under conditions of selective pressure for genetic complementation. Isolation of a fuctional protein from the yeast strain was not reported. The gene for human leukocyte interferon has been expressed in yeast, under control of the yeast ADH1 (alcohol dehydrogenase) promoter. In that instance, successful production of an active protein did not require post-translational processing or assembly of components.
DNA transfer vectors suitable for transfer and replication in yeast have been developed (Broach, J. R. et al., Gene 8, 121 (1979); Hartley, J. L. et al., Nature 286, 860 (1980). Most yeast vectors in current use are derived from E. coli vectors, such as pBR322, into which have been inserted a yeast origin of replication. Two types of yeast replication origins are available. The first, derived from a ubiquitous naturally-occurring yeast plasmid, commonly referred to as the 2 micron circle, confers the ability to replicate independently of yeast chromosomal DNA. Another class of vectors contains a replication origin sequence termed ars1 (autonomous replication sequence), derived from the yeast chromosomal replication origin, which also provides autonomous replication capability. Because both bacterial and yeast replication origins are present in the same vector, they can he used in either organism. Selection may be provided for in bacterial systems by the inclusion of antibiotic resistance genes, such as the ampicillin and tetracycline resistance genes of pBR322. Selection in yeast systems typically may be provided for by including a yeast gene complementing a mutation in a suitable auxotrophic host strain. The studies reported herein conveniently utilize yeast vectors containing a promoter isolated from the yeast gene coding for alcohol dehydrogenase (ADH1). (Bennetzen, J. L. et al., J. Biol. Chem. Vol. 257, p. 3018 (1981).
The ADH1 promoter region was isolated from the 5xe2x80x2-flanking region of the yeast ADH1 gene. A fragment containing approximately 1600 base pairs of the ADH1 sequence extending from position xe2x88x921550 to +17 within the coding region was fused to the yeast CYC1 coding sequence. Studies on transcription of the attached CYC1 coding sequence deomonstrated that transcript starting specificity could be transferred from one yeast gene to another. Smaller fragments, lacking all of the ADH coding region, have subsequently been constructed, and shown to be functional in the expression of human interferon. One such fragment, designated 921, is terminated after position xe2x88x929, and was employed in the present studies.
Because substances reactive with anti-HBsAg antibody exist in several forms, a nomenclature has been adopted herein to distinguish these forms. The translation product of the HBV surface antigen gene is termed the S-protein. S-protein has 226 amino acids whose sequence has been inferred from the nucleotide sequence of its gene and by partial sequence analysis. HBsAg as used herein includes the major surface antigenic component of HBV found in infected patients"" serum and in Alexander cells, a hepatocellular carcinoma cell line which synthesizes and excretes 22 nm HBsAg particles (Alexander, J. J. et al., S. Afr. Med. J. 50, 1124 (1976). Both S-protein and HBsAg are antigenic, however, the latter is more reactive against anti-HBV antibody and is considered more immunogenic. Since the structure of HBsAg is not fully characterized, and the contributions to antigenicity and immunogenicity of various modifying steps not fully understood, the term HBsAg is used herein to include any modified form of the S-protein which contributes to its antigenic and immunogenic properties, including, but not limited to, dimerization, glycosylation, and particle assembly.
The present invention relates to synthesis of HBsAg in yeast. Yeast expression vectors comprising a yeast promoter, ADH1, have been constructed. The region of the HBV genome coding for the S-protein, excluding a possible 163 amino acid presequence, has been transferred to the yeast expression vector.
Using the described yeast vector, the successful synthesis of HBsAg by yeast has been achieved. The product is antigenic (reactive with anti-HBsAg), and a substantial portion is found associated with particles identical in electron microscopic appearance to those found in the serum of HBV-infected patients and in Alexander cells but having a smaller particle size diameter. The HBsAg synthesized by yeast has identical sedimentation behavior to purified, naturally-occurring HBsAg particles purified from Alexander cells as measured by sucrose gradient sedimentation. The present invention demonstrates synthesis and assembly of a higher ordered multi-component structure resulting from expression of a heterologous DNA coding segment in a microorganism.